As various members (for example, oil pan, engine head cover, AT mission case) of a car and the like, recently, molded products made of magnesium or a magnesium alloy are used. When a bolt made of a metal such as iron having a higher electric potential than the above-described members is used to bind the members tightly, an electrolytic corrosion action may occur at a portion where different kinds of metals contact each other and thus the members may corrode. Thus, a method of preventing the electrolytic corrosion by interposing an insulation washer between different kinds of metals to be fixedly bound to each other with a bolt and a method of treating the surface of the bolt or the like to prevent the occurrence of the electrolytic corrosion are conventionally used. In these methods, the washer and the bolt may rub each other, which may result in a decrease of an electrolytic corrosion effect or an electrolytic corrosion prevention film may separate from the surface of the bolt or the like, which may result in a decrease of the electrolytic corrosion effect. In addition, in the bolt (Japanese Patent Application Laid-Open No. 2005-291278: patent document 1), the surface of at least one of the bolt and the washer is treated to prevent it from electrolytically corroding, and the washer is fitted on the bolt body and fixed to the seat surface of the bolt with the washer in contact therewith. There is also proposed the bolt (Japanese Patent Application Laid-Open No. 10-184633: patent document 2) having the flange-provided head part and the molded winding drum part, made of synthetic resin, which is larger than the short flange portion in its diameter and has the short-leg cylindrical portion extended from the seat surface of the flange-provided head part at the lower end thereof. The molded winding drum part is integrated with the peripheral surface of the flange-provided head part.
Patent document 1: Japanese Patent Application Laid-Open No. 2005-291278
Patent document 2: Japanese Patent Application Laid-Open No. 10-184633